The present invention relates to a method for determining the optimum sampling times for a quarature phase shift keyed (QPSK) or quadrature amplitude modulated (QAM) received signal that has been influenced by radio link interferences.
In the transmission of such a phase or amplitude and phase modulated signal over a directional radio path, multipath propagation frequently produces strong interference which make faultfree demodulation more difficult. Due to the multipath propagation, the two quadrature components of the modulation signal experience amplitude as well as phase travel time distortions. There now exists the problem of determining, for the distorted quadrature components of the received signals, sampling times in the demodulator which assure the best possible error free recovery of the original information from the interfered with signal. German Pat. No. 1,512,561 discloses a method according to which sampling occurs always at that time at which one of the quadrature components reaches its maximum. However, due to the amplitude and phase travel time distortions, there are circumstances when this maximum may lie very far away from the sampling value representing the correct information.